Stucco is a masonry mortar applied as a thin coating to an outer building surface. It may be applied by trowel or spray-applied using pressurized equipment. Stucco is made from a special Portland cement known as a masonry cement, which contains a limestone mixed with water and sand.
It is known to incorporate additives into masonry cements to enhance certain properties. For example, U.S. Pat. No. 6,451,880 describes the use of such cement additives to impart air entertainment ability, workability, and board life properties. A certain level of workability in stucco is required to allow, for example, proper hand application using a trowel. As defined in the '880 patent, “workability” means and refers to the consistency and feel of the mortar when it is trowel-applied by a skilled mason. The masons prefer that their mortars have a particular ease of movement, a desirable plasticity or “fattiness” that translates to easy application, as well as a certain degree of “body” or cohesiveness that holds the mortar together. In other words, stucco should remain in place once applied to a surface; it should not fall off, sag, or otherwise move unless physically displaced or shaped by hand trowel.
In addition to having workability and sufficient body, stucco must resist setting up (or hardening) too fast when spray applied using pressurized equipment. Because spray equipment is moved from place to place at a typical construction site, where delays of 10-15 minutes are not uncommon, the stucco material must resist setting in the spray hose and nozzle, or otherwise this expensive equipment could be ruined.
The nature of the sand aggregate used in stucco can adversely affect the workability of the stucco. For example, masonry contractors typically employ a clean sand aggregate that conforms to ASTM C-897 (2000) in order to limit their liability. This sand is considered to be a “clean” type of sand because it contains relatively smaller amounts of very fine particulates. One advantage of this coarser sand is its relatively smaller water demand and consequently smaller shrinkage-cracking feature when compared to finer sands. However, due to its coarser nature, this sand can make the stucco less “workable” from the viewpoint of the masonry contractor.
The “sand carrying capacity” of a masonry mortar refers to the maximum amount of sand that can be added without defeating the workability of the mortar. Use of a masonry cement with a high sand carrying capacity is beneficial. Masons are allowed to prepare masonry mortar or stucco with a 4:1 ratio of sand to cement. Masonry mortar and stucco having high levels of sand have an advantage in decreased costs, lower shrinkage, and lower cracking tendencies, but have a disadvantage in terms of a higher potential for brittleness.
Masons are usually not able to realize the economic benefits of employing a 4:1 ratio in their mix designs, however, due to factors that limit the workability of their mortar cements, such as the coarseness of sand as just mentioned. The ratio of sand to cement is more often limited to below 2:1 due to the limited workability of the stucco.
To remedy limitations in stucco workability, masons can increase the levels of particle fineness in their mortar cement. There are several approaches to accomplishing this. For example, the cement portion can be increased, but this can increase costs and the tendency for shrinkage and cracking, as previously mentioned. Another possibility is to incorporate bentonite and other clays to increase the portion of the fines, but this may be undesirable if the final physical properties of the resultant stucco are changed.
Certain high performance masonry cement additives can be used to increase workability of masonry cement. However, performance may not be optimized for a stucco application with ASTM graded sands wherein a high level of adherence and a high sand carrying capacity is required. Thickeners can be used to increase water retention, workability, and board life of masonry cement. For example, U.S. Pat. No. 6,451,880 disclosed the use of cellulose ethers.
In view of the prior art, it is believed that a novel cement additive and process is needed for modifying the rheological qualities of masonry cement and stucco, and in particular stucco mortars prepared with coarse ASTM graded sand.
In surmounting the disadvantages of the prior art, the present invention provides a cement additive that employs an extra-cellular biopolymer in combination with an anionic surfactant and amphoteric or cationic surfactant to provide mortar cements having desired rheological properties for stucco prepared with coarse sand.
While diutan S-657 biopolymer has previously been used in cementitious compositions, the present inventors believe that such applications do not involve highly entrained air as in the presently described stucco applications. For example, published US patent application No. 20060280970 A1 disclosed grout compositions in which defoamers were used to reduce air bubbles formed during mixing of the composition with water, such that high strength flooring could be achieved. As another example, published US patent application No. US 2004072939 A1 (of Cornman et al.) taught that both foamers and anti-foaming agents can be used to stabilize the formulation using diutan S-657 biopolymer among others, but this reference did not focus on entraining air alone for stucco applications, and in fact teaches that it is preferable to use defoaming agents, such as tributyl phosphate (See e.g., Paragraph “0039”).